Liquid membrane separation has its origins in liquid-liquid extraction which involves extraction of a solute from a first liquid using a second liquid solvent that is essentially immiscible with the first liquid. A back extraction is then typically done in a separate apparatus to remove the solute from the second liquid. Liquid-liquid extraction can be carried out in a number of devices such as mixer settlers, packed towers, bubble tray columns, asymmetrical membranes, and supported liquid membranes. Of all of these types of liquid-liquid extraction, supported liquid membrane technology belongs to a separate category of extraction that involves the presence of a carrier that forms a reversible complex with the preselected chemical species of interest.
Although other types of liquid-liquid extractions can be carried out using membrane supports, such processes are clearly distinguishable from supported liquid membrane transport in that such methods lack the presence of a carrier that forms a reversible complex to facilitate transport of the preselected chemical species. Such processes do not involve the formation of a reversible chemical complex but rather the physical partitioning of the solute between the two immiscible liquids. As a result, the rate of transport is very slow as the transport involves no reaction to form a chemical complex. Fluxes are low and the devices tend to be large. To facilitate liquid-liquid contact while preserving membrane life, such devices typically use asymmetrical supports such as membranes that are hydrophilic on one side and hydrophobic on the other side or membranes in which a gradient of pore size and density are used.
Liquid membrane transport separation is an emerging technology where specific material species are transported selectively and rapidly across a liquid membrane. Though liquid membrane transport was discovered in the early 1970s, the bulk of experimental studies involving this technique has been carried out only in the last few years.
In spite of the success of supported liquid membrane techniques in the laboratory, very few pilot scale studies have been undertaken. The primary reason for this low utilization has been the low flux rates and membrane instability. The high surface area per unit volume of hollow fiber microporous support members has increased the flux rate to some extent but flux rate improvements are still necessary to achieve commercial utilization. An even greater problem is membrane instability which generally occurs because of the gradual loss of the liquid membrane to the liquids on each side of the membrane. Such loss can occur because of (1) the solubility of the carrier and its diluent in the feed and strip liquids or (2) capillary displacement as a result of an osmotic pressure differential between the two sides of the membrane due to solution pumping of the feed and strip liquids.
Liquid membrane instability is a prime causes for the slow commercialization of the process. Although considerable research has been conducted to determine the causes for membrane instability, a permanent remedial solution has yet to be found. Studies as to osmotic pressure differences, interfacial membrane-solution tension, and low critical surface tension of the polymer support have been inconclusive.
Generally supported liquid membrane instability is subdivided into (1) instability arising from loss of carrier liquid (extractant) from the membrane phase leading to a loss in permeability, or (2) a complete "break down," resulting in direct contact of the feed and strip liquids. Attempts have been made to explain the complete "break down" effect on the basis of the osmotic pressure gradients across the membrane. Feed and strip liquid transport increases with this gradient, and this transport induces a repulsion of the liquid membrane phase out of the pore of the support which causes the membrane to degrade (Fabani, C., et al., J. Membrane Science, vol. 30, p. 97, 1987). Fabani concluded that osmotic pressure differences play a significant role in determining membrane stability and, as a result of these osmotic pressure differences, the carrier liquid (extractant) and its diluent is washed from the pores of the membrane. In direct contradiction, Danesi, P.R., et al., J. Phys. Chem., vol. 1, p. 1412 (1987) concluded that the "osmotic pressure model" was not responsible for membrane instability. He and Takeuchi, et al., J. Membrane Science, vol. 34, p. 19 (1987) postulated that this form of instability was due to the poor nature of the carrier liquid diluent and low carrier liquid/diluent feed and strip interfacial tensions.
Takeuchi (1987) also found that membrane instability increased with feed velocity and also with increasing hydrostatic pressure gradient across the membrane. Neplenbroeck (1987) concluded that membrane instability depended on the type of feed and strip liquids used and the molecular structure of the carrier. He concluded that there was no direct relation between viscosity of the carrier and diluent and membrane instability but rather that there was a connection with the interfacial tension of the carrier diluent and the feed and strip liquids. However, these relations are ambiguous leaving strong doubt that membrane instability is not caused by osmotic pressure differences.
Other researchers such as Chirazia (1990) and Takeuchi (1987) have suggested that the use of amines as membrane diluents leads to better stability when compared to aliphatic diluents, that the degree of amine solubility in feed and strip solutions contributes to membrane stability, that interfacial tension lowering at the carrier-diluent/feed solution and strip liquid interface increases membrane stability, that the use of polymers which can cross link the carrier to the support can increase membrane stability, that the use of hollow fiber modules rather than flat sheet modules and membrane support pore size has an effect on membrane stability, and that the surface tension of the support in relation to the carrier-diluent/feed and strip interfacial tensions affect membrane stability. Dozol, J.F., et al., J. Memb. Sci., vol. 82, p. 237 (1993) suggested that the solvent solubility of the solvent and the simultaneous drop point of the membrane as proportional to the interfacial tension could effect membrane stability. They felt that the liquid membrane must have surface tension lower than the critical surface tension of the support. They also concluded that the carrier solution viscosity has no effect on membrane stability. In view of these significant differences as to the causes of liquid membrane instability and inconclusive experimental results, there are significant doubts in the minds of researchers regarding the real cause of membrane instability. As such, a viable solution to the membrane stability problem has yet to be found and commercialization of this technique has yet to be achieved.